Variable Volume Fed-batch Reactors
In the variable volume fed-batch reactor, volume changes with fermentation time due to the substrate feed. The volume changes can be controlled depending on the requirements, fermentation kinetics and characteristics of microorganisms. In this type of fed-batch reactor liquid volume can also be used as a variable in the optimization of the product yield. For example, when fermentation reached a certain stage after which it is not effective anymore, a quantity of culture can be removed from the vessel and replaced by fresh nutrient medium. The decrease in volume results in an increase in the specific growth rate, followed by a gradual decrease as the quasi-steady state is established.
Then there can be variable fed-batch reactors in which supplementary growth medium is added during the fermentation, but no culture is removed until the end of the batch. This system presents a disadvantage over the fixed volume fed-batch and the repeated fed-batch process; much of the fermenter volume is not utilized until the end of the batch and, consequently, the duration of the batch is limited by the fermenter volume. A control system is an essential part of the variable volume fed-batch reactor.